Organic electroluminescent elements have attracted attention as a next-generation illumination light source because they enable planar light-emission and enable light emission of any color tone depending on the selection of light-emitting substances, and have been vigorously developed with the aim of practical use. In particular, research and development has been actively conducted especially with regard to technologies for improving color rendering properties, which is regarded as one of problems with inorganic LED lighting, and methods for improving color rendering properties by various design and technological developments of devices have been proposed. However, problems still remain in order to realize sufficient improvement in color rendering properties in comparison with conventional main lighting, that is, fluorescent lamps.
In JP 2009-224274A (Patent Literature 1), a method is proposed for improving color rendering properties by stacking a blue-region light-emitting layer having the maximum emission wavelength in a wavelength region between 450 nm and 470 nm, a yellow-region light-emitting layer having a maximum emission wavelength in the wavelength region between 550 nm and 570 nm and a red-region light-emitting layer having the maximum emission wavelength in a wavelength region between 600 nm and 620 nm. High color rendering properties in which an average color rendering index (Ra) is 80 or more, can be realized by thus stacking three light-emitting layers whose emission wavelength regions have been appropriately selected, but Ra of a general fluorescent lamp is from 80 to 90, and problems still remain as a method for realizing sufficiently high color rendering properties in comparison with conventional light sources. Furthermore, a method for improving a special color rendering index R9 for red, which is a problem with inorganic LED lighting and is important to general lighting, is not referred to.
Furthermore, in JP 2007-189002A (Patent Literature 2), a method is proposed for realizing high color rendering properties by using two kinds of blue light-emitting materials having different maximum emission wavelengths, with an emission peak of the blue light-emitting material on a short wavelength side of the spectrum being between 430 nm and 465 nm and an emission peak of a blue light-emitting material on a long wavelength side of the spectrum being between 465 nm and 485 nm, and by using a fluorescent light-emitting material as the blue light-emitting material on the short wavelength side of the spectrum and a phosphorescent light-emitting material as the blue light-emitting material on the long wavelength side of the spectrum. However, there is a problem with the life property of phosphorescent light-emitting materials having an emission peak in a range of 465 nm to 485 nm in comparison with fluorescent light-emitting materials having an emission peak in the same wavelength region, and a material having a high triplet energy level needs to be used as a peripheral material in order to allow such a blue light-emitting material on the short wavelength side of the spectrum to efficiently emit light, thus limiting the device structure. Furthermore, color rendering indices which are important indicators as a method for evaluating the color rendering properties of a light source for lighting applications are not referred to and a method for improving color rendering properties is not clearly defined.
Furthermore, in JP 2006-287154A (Patent Literature 3), a method is proposed for realizing high color rendering properties by having maximum emission wavelengths in the respective regions of 440 nm to 480 nm, 510 nm to 540 nm and 600 nm to 640 nm and setting the minimum emission intensity between the maximum emission wavelengths to be not less than 50% of the maximum emission intensity in the adjacent wavelength region. However, in reality, there are cases where Ra is low even if the minimum emission intensity between the maximum emission wavelengths is not less than 50% of the maximum emission intensity in the adjacent wavelength region and, conversely, it is possible to improve Ra to 90 or more even if a minimum emission intensity between the maximum emission wavelengths is not more than 50% of a maximum emission intensity in the adjacent wavelength region, and thus problems remain as a method for improving color rendering properties. Furthermore, color rendering indices which are important indicators as a method for evaluating color rendering properties of a light source for lighting applications are not referred to and it is also difficult to say that a method for improving color rendering properties is clearly defined in this respect.
Furthermore, in WO2008/120611 A1 (Patent Literature 4), a method is proposed for realizing high color rendering properties by using a light-emitting layer constituted by not more than three layers and using at least four kinds of light-emitting materials having respectively different maximum emission wavelengths as light-emitting materials contained in the light-emitting layer. By using four kinds of light-emitting materials having different maximum emission wavelengths, it is possible to improve Ra to more than 70, but, as described above, Ra of a general fluorescent lamp is from 80 to 90, and thus problems remain as a method for sufficiently improving color rendering properties in comparison with conventional light sources.